Stable and Specific Tracers

Abstract

Since the introduction of labeled dexamethasone to detect and characterize the glucocorticoid receptor, the use of labeled synthetic hormones as tracers has become increasingly widespread. The progestin receptor is now rarely assayed with labeled progesterone, but with radioligands such as [³H]promegestone (Duffy and Duffy, 1979; McGuire et al., 1977b; Horwitz and McGuire, 1975; Philibert and Raynaud, 1973, 1974a, b) and [³H]-ORG 2058 (Keightley, 1979; Koenders et al.,1979; Jänne et al., 1978; Fleischmann and Beato, 1978); the androgen receptor is assayed with [³H]metribolone rather than labeled dihydrotestosterone (Raynaud et al., 1980a; Ekman et al.,1979b; Ghanadian et al., 1978; Menon et al.,1978; Shain and Boesel, 1978; Cowan et al.,1977; Asselin et al.,1976a; Bonne and Raynaud, 1975, 1976), and the estrogen receptor can, in the opinion of several authors, often be unambiguously detected only with ligands such as [³H]moxestrol (MacLusky et al., 1979a, b; Martin et al.,1978; Okret et al., 1978; Raynaud et al.,1978; Raynaud and Moguilewsky, 1976, 1977; McEwen et al.,1975; Raynaud, 1974). The reasons underlying the ever-increasing use of these synthetic radioligands (tags) are to be found in the distinct advantages they offer over labeled natural hormones as described previously in some detail (Raynaud et al., 1979b) and as briefly summarized herein. Unlike the natural hormones, tags do not bind with high affinity to the plasma proteins that often contaminate tissue and cytosol preparations (Ballard, 1979; Hähnel and Twaddle, 1979; Krieg et al., 1979; Karr et al., 1978; Wagner, 1978). Unlike estradiol, testosterone, and dihydrotestosterone, neither moxestrol nor metribolone binds extensively to human sex-steroid-binding protein (SBP); unlike progesterone, hydrocortisone, and aldosterone, neither promegestone nor dexamethasone binds with high affinity to human corticosteroid-binding globulin (CBG). Furthermore, tags are resistant to degradation and form more stable (slowly dissociating) receptor complexes than the natural hormones, thus facilitating the detection of receptors and enabling their measurement by exchange assay. For instance, metribolone, unlike dihydrotestosterone, is not degraded on in vitro incubation with cytosol (Raynaud et al., 1979b; Krieg et al., 1979; Pousette et al., 1979; Tremblay et al., 1977), and both promegestone and dexamethasone form cytosolic receptor complexes that dissociate considerably more slowly than the corresponding progesterone complex (Raynaud et al., 1979b; Keightley, 1979; Jänne et al., 1978; Moguilewsky and Raynaud, 1977; Philibert and Raynaud, 1977) and hydrocortisone complex (Munck and Leung, 1977; Rousseau et al., 1972). Finally, if possible, tags should be more receptor-specific than the natural hormones. Hydrocortisone and aldosterone bind to at least two different tissue receptors, a mineralocorticoid and a glucocorticoid receptor, that are not easy to distinguish in the absence of ultraspecific radioligands (Raynaud et al., 1979a; Matulich et al., 1976; Rousseau et al., 1972).